The present invention relates generally to an improved method for acquiring magnetic resonance images (MRI) of blood-carrying vessels, and more particularly to, a method and apparatus to acquire post-contrast images to visualize arterial and venous structures that is not time dependent on the acquisition of the images during the arterial or immediate post-injection period of a contrast bolus and uses steady-state free precession (SSFP) pulse sequences.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment My. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. Magnetic resonance angiography (MRA) is an emerging technology for the non-invasive assessment of arterial and venous structures. Intravenously administered contrast agents increase the visualization of these structures for contrast-enhanced MRA, especially if performed during their initial passage (a.k.a. first pass or arterial phase) in the target vessel. The imaging of small vessels with contrast-enhanced MRA techniques however requires a compromise of image spatial resolution and imaging time. In general, higher spatial resolution requires extended imaging time. Extended imaging time, however, diminishes the ability to achieve high arterial signal-to-noise (S/N) since bolus duration must be extended to match the elongated imaging time. This is problematic since a slower bolus administration results in lower achieved concentrations of contrast media. In addition, by prolonging the acquisition period, the signal intensity of the venous structures is increased due to venous re-circulation. Therefore, extending or prolonging data acquisition after the initial contrast passage will lead to not only diminished arterial signal intensity but also increased venous signal intensity, the result of which is compromised ability to visualize diseased vessels.
In order to improve vascular depiction, background suppression is usually employed. Background suppression for MRA is obtained by either applying fat suppression pulses during the first pass acquisition, or subtracting out the background signals using a pre-contrast mask. The pre-contrast mask image has identical acquisition parameters as the first pass acquisition. Signal intensity for all structures in the pre-contrast mask image is identical to that of the first pass acquisition, except for the vascular structures. Subtracting the pre-contrast mask from the first pass image then yields only signal from vascular structures. This is particularly important in imaging the small vessels such as may exist in the calf region in a patient because of the amount of fat and soft tissue background which often obscures adequate visualization of the small vessels. However, there are problems with both techniques. For example, applying fat suppression pulses is dependent on good magnetic field homogeneity and tends to increase the overall scan time. This increased scan time also increases the possibility of venous signal contamination and suboptimal arterial signal. The technique of using a mask subtraction requires that the patient not move significantly between the mask acquisition and the contrast-enhanced acquisition. Failure to do so will cause mis-registration artifacts in the reconstructed image, thereby resulting in inadequate background suppression and artifacts from the subtraction of mis-registered objects.
Coherent steady-state free precession (SSFP) is a technique in which the free induction decay (FID) signal (i.e., S+ signal) and the spin echo signal (Sxe2x88x92 signal) from a train of RF pulses are refocused within each repetition time (TR) interval. The zeroth gradient moments accumulate to zero at the end of each TR interval. This results in the same amount of transverse and longitudinal magnetization being generated after each radio frequency (rf) pulse and increases the available image signal-to-noise ratio (S/N). However, use of this contrast acquisition technique results in high signal intensities from fat which do not decrease significantly with increasing flip angles. Furthermore, as the tissue contrast is a function of the ratio of the spin-spin relaxation time (T2) and the spin-lattice relaxation time (T1) high signal intensity is also returned from fluid. Subsequently, high signal intensity is obtained from the internal abdominal organs. This significant drawback has prevented coherent SSFP acquisition from being used with first pass MRA since there is but one chance to acquire a good image. Furthermore, coherent SSFP acquisition results in high signal intensities from water as well. Therefore, the signal from the small bowel, bladder, etc., remains high in both the post and pre-contrast SSFP images which diminish its application for imaging vessels in the abdomen and pelvis. Since the motion from these structures cannot be predicted and is fairly random, subtraction cannot effectively suppress signals from these structures.
On the other hand, coherent SSFP images have higher image S/N than conventional gradient echo images as used for conventional contrast-enhanced MRA. These S/N improvements of SSFP can also improve visualization of vessels having slow or disrupted flow as seen in areas of stenosis or intimal pathology circumstances which often result in the over-estimation of disease using conventional contrast-enhanced MRA methods. It would therefore be desirable to have a technique, or a series of techniques, to maximize the use of SSFP for MRA applications that do not require the use of a pre-contrast mask image, and is therefore not as sensitive to the time relative to the administration of a contrast bolus, nor spatially limited by the same temporal considerations of bolus kinetics.
The present invention relates to a system and method for acquiring MR images using SSFP pulse sequences to produce images with high background suppression and significant contrast, such as between vessels and adjacent soft tissue, such as fat, muscle, bone marrow, fluid, and such, that solves the aforementioned problems.
Since it is known that the coherent SSFP technique results in an acquisition with high signal intensity for blood, water, and fat, the present invention takes advantage of this result by repeating such an acquisition with an additional Sxe2x88x92 SSFP acquisition in which structures with moving blood have relatively low signal intensity. The Sxe2x88x92 SSFP acquisition is an incoherent steady-state technique that acquires signal only from the FID that is refocused by the subsequent rf pulse. Thus, tissue contrast is a function of T2 and it has similar signal intensity characteristics as the coherent SSFP signal. The exception is that by spoiling the FID, the refocused echo is highly sensitive to flow related dephasing, leading to dark signal in vascular structures. By subtracting the Sxe2x88x92 SSFP image from the SSFP image, a high image S/N of preferentially arterial and venous structures can be obtained. This is possible even where both images are acquired a considerable amount of time after the first pass of the contrast agent. In addition, the image S/N of the arterial and venous signal is much higher than that encountered using conventional gradient echo techniques performed at the same delayed time following the administration of contrast media.
Further, since the signal from vascular structures is relatively low with black-blood image contrast in the Sxe2x88x92 SSFP images, better background suppression can be realized if a scaling factor, or weighting, is applied to the Sxe2x88x92 SSFP image to improve vessel conspicuity. This technique is particularly useful for screening of vascular pathology, especially venous vascular disease, such as lower extremity deep venous thrombosis that can occur over an extensive anatomic region that spans from the calf through the inferior vena cava, right ventricle and into the pulmonary arteries.
There are a number of advantages to using such a technique. For one, the image S/N is higher than that of conventional gradient echo images. Another is that since the mask and the SSFP image can be acquired back-to-back, the patient is required to remain still for only a very short time, which significantly reduces the occurrence of spatial mis-registration artifacts. Further, the ability to obtain MRA images well after the first pass of the contrast agents allows improved visualization of the arterial and venous structures with higher spatial resolution. Such delayed imaging also provides a reliable backup sequence in case the primary angiographic sequence fails.
In accordance with one aspect of the invention, an MR imaging technique includes injecting a contrast bolus into a patient, and then applying a pulse sequence with refocusing Sxe2x88x92 signals from a train of RF pulses to a desired FOV in the patient at a time that is independent of when the contrast bolus was injected.
The technique includes acquiring both an Sxe2x88x92 SSFP mask image and an SSFP image, and then subtracting the Sxe2x88x92 mask image from the SSFP image. The mask image can be acquired either before or after the primary SSFP image is acquired, or alternatively, a mask image can be acquired both before and after the primary SSFP image in order to minimize the effects of patient motion. In the latter case, either the system or an operator can select the better mask image to use in the subtraction part of the imaging process. An optional scale factor can be applied to the mask image to further improve background tissue suppression. A hybrid approach is also disclosed in which a gradient echo pulse sequence is applied according to conventionally known first-pass MRI techniques to visualize the larger arterial vessels, and then an SSFP sequence is applied to visualize the smaller vessels.
In accordance with another aspect of the invention, an MRI apparatus to acquire MR angiography (MRA) images includes a magnetic resonance imaging system having an RF transceiver system and a plurality of gradient coils positioned about the bore of a magnet to impress a polarizing magnetic field. An RF switch is controlled by a pulse module to transmit RF signals to an RF coil assembly for acquiring MR images. A computer is programmed to acquire the SSFP and the Sxe2x88x92 SSFP images. It is noted that either image can be acquired first, and preferably, the images are acquired with minimal time separation therebetween to reduce spatial mis-registration artifacts secondary to patient motion. The computer then subtracts the Sxe2x88x92 mask image from the SSFP image and reconstructs an image from the subtracted images having a high contrast and significant background suppression. This technique is equally applicable to both 2D images and 3D volume acquisitions.
Yet another aspect of the invention includes a computer program having a set of instructions which, when executed by a computer, cause the computer to apply an SSFP pulse sequence and acquire an SSFP image, and apply an Sxe2x88x92 SSFP pulse sequence and acquire an Sxe2x88x92 SSFP image. The Sxe2x88x92 SSFP is then subtracted from the SSFP image and an image is reconstructed with high background suppression and significant contrast between vessels and the background, such as fat, muscle, bone marrow, fluid and other soft tissue.
This technique allows the imaging of arterial and venous structures in a delayed period, or xe2x80x9cquasi-steady-statexe2x80x9d following the administration of a contrast media. Unlike conventional MR techniques, the mask image can be acquired at any time, and is not dependent on the time of administration of the contrast media. This technique can also be combined with conventional first-pass MRA that uses a gradient echo pulse sequence to image large vessels while the SSFP sequence can be used to visualize the smaller or more distal vessels.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.